1. Field of the Invention
The present invention relates to a technique for controlling a variable optical filter whose transmissivity is changed periodically along an optical frequency axis, and in particular, relates to a variable optical filter suitable for compensating for a tilt due to wavelength (or optical frequency) dependence of an optical transmission path, an optical amplifier and the like, an optical transmission system using the same, and a method of controlling the variable optical filter, following the variation of the tilt.
2. Description of the Related Art
Heretofore, it has been known that, in a wavelength division multiplexing optical transmission system for transmitting a wavelength division multiplexed (WDM) signal light through an optical amplifying and repeating transmission path which is configured by connecting an optical fiber transmission path and optical amplifying repeaters, deviation of signal light level due to wavelength (or optical frequency) dependence of a gain of the optical amplifying repeater and the like is compensated for using an optical filter (referred to hereunder as a variable optical filter) with a variable transmission characteristic.
As the variable optical filter utilized for compensation as described above, there have been known a variable optical filter using a single periodic filter whose transmissivity is changed periodically along an optical frequency axis, one in which a plurality of periodic filters with mutually different periods is connected in series to construct a filter characteristic of desired shape along the optical frequency axis based on the theory of Fourier series expansion, and the like (refer to Japanese Unexamined Patent Publication No. 6-276154, Japanese Unexamined Patent Publication No. 9-244079, Japanese Unexamined Patent Publication No. 9-289349, Japanese Unexamined Patent Publication No. 2000-199880). In the variable optical filter with such a periodic filter characteristic, the variable filter characteristic thereof is controlled according to the variation of required filter characteristic, thereby enabling to compensate for the deviation of signal light level over a wide range.
Incidentally, for a conventional variable optical filter with the periodic filter characteristic as described above, in the case where the required filter characteristic continues to be varied slightly in an optical frequency axis direction, if a range of the variation is within a variable range of filter characteristic, it is possible to follow the required filter characteristic by changing the filter characteristic minutely at each variation of the required filter characteristic. However, if the range of the variation is outside the variable range of filter characteristic, even if an amount of the variation at that point is minute, it is not possible to continuously change the filter characteristic in response to the requirement.
In order to comply with the requirement in such a situation, for example, it is necessary to control the periodic filter characteristic to be back by at least one period in an opposite direction from a required changing direction. In this case, even if the change of the filter characteristic in the vicinity of the control is minute, since sometimes the filter characteristic must be varied significantly during the control, there is caused a problem in that the light intensity may be varied significantly over the whole optical frequency band, being a compensation object.
Here is a specific description of the above problem using FIG. 9 and FIG. 10. In the following description of the periodic filter characteristic of a variable optical filter, focusing attention on one attenuation peak where the attenuation is maximum, as a reference peak Pref. Then, the variable range of filter characteristic in the optical frequency axis direction is designated a variable range R of the above described reference peak Pref.
The consideration is made on, for example, the case where, when a state A shown at the top of FIG. 9 is an initial state of the variable optical filter, the filter characteristic is required to move continuously by each minute amount in parallel to the frequency axis direction toward a high frequency side. In this case, the variable filter characteristic can follow from the state A through a state B and a state C up to a state D. However, when the variable filter characteristic is required to be changed to a state E as shown at the bottom of FIG. 9, even if a difference between the filter characteristic in the state D and the filter characteristic in the state E is minute, since the change of the filter characteristic exceeds the variable range R, it is not possible for the reference peak Pref in the state D to continue to move to a location α. Therefore, in order to change the filter characteristic from the state D to the state E, it is necessary to move the reference peak Pref to a location β.
For an operation of when the filter characteristic is changed from the state D to the state E by moving the reference peak Pref to the location β, it is possible to consider the case where the filter characteristic is changed from a state F to a state H via a state G, or the case where the filter characteristic is changed from the state F to the state H via a state I, as a more specific example.
In the case where the filter characteristic is changed via the state G, since the reference peak Pref crosses a region where the reference peak Pref is not primarily required to have a peak characteristic, the large unnecessary variation occurs in the filter characteristic during the change.
On the other hand, in the case where the filter characteristic is set to be a flat transmission state with respect to the optical frequency as shown in the state I, the reference peak Pref is moved to a location of the reference peak Pref in the state H while maintaining the state I, and thereafter, the attenuation of the variable optical filter is increased, the peak characteristic primarily required disappears momentarily over the whole optical frequency band being the compensation object, while the filter characteristic is maintained in the state I. Therefore, the large unnecessary variation occurs in the filter characteristic.